The evolutionary changes of erythrocyte from sea to land : cytological features and sites of development

The evolutionary story of erythrocyte starts in a very ancient period and proceeds through phylogenetic changes, regarding its origin, structure and hemoglobin content. The earliest hemoglobin- bearing cell becomes apparent in a limited number of marine invertebrates, as a coelomocyte carrying hemoglobin. The occurrence of erythrocytes during evolution allows an increasing concentration of hemoglobin, vehiculated in cells rather than free in hemolymph/coelomic solution, simplifying its structure that otherwise should be polymeric, to avoid its free diffusion out of hemolymph. Erythrocytes of invertebrates and submammalian vertebrates, with rare exceptions, are nucleated cells, larger than human red blood cells, and oval or ellipsoidal or discoidal, rarely spheroidal, in shape. These early erythrocytes contain less hemoglobin per unit volume than the mammalian erythrocyte. Erythrocytes of invertebrates mature into circulation through stages similar to those of submammalian vertebrates. Circulating erythropoiesis is a peculiarity that will be conserved in all submammalian vertebrates. The definitive structure of the submammalian vertebrate erythrocyte occurs in fishes whose erythrocyte feature, mainly ellipsoidal, biconvex and nucleated, is conserved through all the submammals. In all Vertebrates, there is an inverse proportion between size and number of erythrocytes observed in all vertebrates including mammals, with the evolutionary advantage of ensuring the best conditions for gas exchange in active metabolic organisms. The sites of origin of erythrocytes in embryonic fishes are located in yolk sac and/or in intermediate cell mass while in adulthood their localization ranges from the presplenic tissue of cyclostomes, to the splenic tissue scattered in the submucosa of gut of Hagfish, to the neural body of adult lamprey - in which blast cells proliferate between adipocytes, as well as it occurs in bone marrow of mammals - to the spleen in elasmobranchs and to the kidneys in teleosts. Nevertheless erythropoiesis is generally located in separate sites from those of granulocytopoiesis in most fishes, in all urodela, both are vertebrates lacking bone marrow. The early bone marrow, during evolution, occurs in Plethodontidae, the family of lungless salamanders, with the sole lympho-granulopoietic function. The phylogenetic onset of medullary erythropoiesis in the Anurans is a hematologic landmark, due, very likely, to protective shielding from ionizing radiation for the radiosensitive hemopoietic stem cells, offered by the bone along with evolutionary transition from water to land. The bone marrow becomes the dominant hemopoietic site in Reptiles and the sole site ion Birds and Mammals, with rare exceptions. Erythropoietic activity, such as all hemopoiesis, in all ecthothermic animals depends on the seasons, as it is more active in warm season and during reproduction period.